How I fixed the P0420 code on my 1998 Honda Accord EX

Is your check engine light on with a code P0420 and you don’t know what to do? Then you probably find yourself in the same situation I did a few months ago. While driving my 1998 Honda Accord, I noticed that the check engine light came on so I took it to the auto parts store to have the code read for free. They said I had a P0420 code which meant “catalytic converter efficiency blow threshold.” So, I looked on the internet and found lots of information scattered everywhere but no real answer on what to do.

After doing a lot of research, I developed a list of things to check. I wanted to make sure I ruled out all the inexpensive and simple options before replacing a major component. This is the list I came up with and I hope you find it helpful. I must let you know that I am not a mechanic and I have no training in this area however I am a fairly handy person and I did a good deal of research on this topic. Also, I make specific reference to my 1998 Honda Accord EX but I believe this list generally applies to most cars.

Before you do any work check to see if your car is still under warranty. According to federal law all catalytic converts have an eight year or 80,000 mile warranty. Additionally, Honda agreed to extend the warranty on emissions components to 14 years or 150,000 miles on most 95-97 vehicles. The lest most inexpensive option is to have it serviced under warranty!

For more information on how the emissions system works see these links:
http://en.wikipedia.org/wiki/Catalytic_converter
http://www.troublecodes.net/articles/catfailure/
http://www.mr2.com/TEXT/O2_Sensor.html

Step 1: Buy the Manual.
If you have spent anytime on forums asking questions the first response you get is always “get the manual.” This is good advice. Throughout this blog I make reference to the Hayne’s Manual for my 1998 Accord. So if you want to follow along and do the work you will need it.


Step 2: Find Other Symptoms.
To accurately diagnose and fix the problem you need as much information as possible. Have you noticed anything different about the way your car runs? Here are some common symptoms

Loss of power: this may indicate your catalytic converter is clogged and must be replaced. However, you should confirm this with a vacuum pressure check before spending the big bucks on a new converter (see step 4).

Blue tailpipe emissions: this may mean that your engine is burning coolant which can ruin the catalytic converter. Check your coolant level every other day for a week or two and see if the level gets lower and lower. Unfortunately, if your engine burns coolant you must fix this problem first and then replace the catalytic converter. If you don’t fix the coolant leak the new converter will suffer the same fate as the last one.

Keep in mind, however, that these symptoms don’t necessarily mean your converter is blocked or your engine is leaking coolant. You must confirm with other tests like the vacuum pressure check and coolant level check. Don’t assume the worst most expensive option rather exclude less expensive options first.

Fortunately my car didn’t have either of these problems!

Step 3: Reset the Check Engine Light.
Resetting the Check Engine Light is the easiest solution. Sometimes your car’s computer (ECM) simply freaks out and gives you this code when there is no real problem. So before going any further reset the ECM and see if the Check Engine Light comes back on. You should also reset the code after you make any repairs. If it doesn’t come back on, your finished!

To reset the light, open the passenger side door and look for a panel on the far right hand side of the dash. The panel is covered by the door when the door is closed. After popping out the panel, remove the fuse in the number 13 slot for at least 10 seconds. The numbers on the fuse themselves represent the load the fuse can handle without blowing not the slot number. The slot number is printed in between the rows of fuses (see pictures below).

Keep in mind, however, that you can not reset the code and immediately take the emissions test. You wont pass. The computer must show that your catalytic converter came on and functioned properly. For the converter to reach light off temperature normally requires some combination of city and highway driving. So take your car for a drive on the interstate for 30min first.





Step 4: Component Check.
Now you need to check the various components of the system to see if any aren’t working properly.

First, inspect the exhaust system (CAUTION: let the car cool for a least 3 hours before doing this inspection and make sure it is securely on safety stands or ramps). Sometimes a leak or hole between the two O2 sensors can allow outside air to mix with the exhaust gases causing a false reading and setting this code. Look for discolored areas where parts are joined together, excessive rust, loose connections, and bent/crushed pipes. Some rust is normal. Then run your hand along the pipes feeling for any holes. Repair as necessary. Also, knock on the converter with your knuckles. If you hear rattling inside the converter then it is damaged and must be replaced.

Second, check the O2 sensors by following the Hayne’s Manuel. You normally check for three things 1)the sensor switches out of open loop—it rapidly switches voltage between 0.2v and 0.8v 2) the sensor has the proper resistance 3)the sensor is getting enough power i.e. 12.5v where it is connected. The first two problems mean a bad sensor the third indicates an electrical problem.

Sensors slowly deteriorate and become sluggish so that they don’t switch voltages as quickly as they should. This sluggishness can cause the P0420 code instead of a bad sensor code because the sensors works well enough to pass the sensor test but don’t match up with each other. My Honda uses a 4 wire preheated 02 sensor which has an expected life of about 100,000 miles. My car had 133,000 miles so I put in two new sensors which fixed the problem. So if your car has 85,000+ miles on it, you should switch out the sensors before going any further (I used Boshe universal O2 sensors which cost $70 each; OEM sensors run $100+).

Front sensor:


Rear sensor in the catalytic converter:


Connection for the rear sensor:


Third, check the electrical system (if both 02 sensors have the correct voltage at the connection you can skip this step). If one O2 sensor has the correct voltage but the other doesn’t, check the wire and connection between the sensor and ECU. Otherwise, check to see if you battery is fully charged and that your alternator is working properly. Most parts stores have a machine that can test the alternator without removing it from the car.


Step 5: Check the Catalytic Converter.
If you have gotten this far your converter has probably gone bad. You have ruled out most other causes and you may opt to go ahead and replace the converter without further testing. However, it can still be a good idea to confirm your findings. There are two possible problems and two ways to check.

A blocked converter: following the instructions in the Hanye’s manual hook up the vacuum gauge and note the pressure (I bought a vacuum gauge for $25). Then rev the engine four times. If the pressure after revving the engine is more than 1 UNIT below the pressure at idol, some component of your exhaust system is clogged. You can then disconnect the converter on the muffler side and letting it hang down perform the test again. If it passes, the converter is not clogged. Otherwise, totally disconnect the converter and do the test. If the car passes the test with the converter disconnected but not with it connected you know that the converter is clogged and must be replaced.

A poisoned converter: unfortunately checking to see if the converter actually converts gases requires thousands of dollars of equipment. Many shops may charge you as much as $100 to do the test. If you have ruled out every other option, you should save your $100 and uses it towards the price of a new converter.

You can spend wildly different amounts on a new converter. I direct replacement from the dealer for my 1998 accord was $675 just for parts. However, I found an aftermarket direct fit replacement for $220. You can buy universal converters for even less but if you don’t have welding equipment, you wont get very far.

LEGAL NOTICE: All this information is for educational purposes only. I assume no responsibility for any repairs or damages. You assume full risk and responsibility should you decide to act on any of this material.